Merge accept4() test from trunk:

r832904 | jorton

* configure.in: Simplify check for accept4().

r1539374 | sf

Use fcntl instead of SOCK_NONBLOCK to make socket nonblocking
in accept4 check.

r1539389 | sf

Fix fcntl(fd, F_GETFL) call in r1539374



git-svn-id: https://svn.apache.org/repos/asf/apr/apr/branches/1.5.x@1540667 13f79535-47bb-0310-9956-ffa450edef68